The present invention relates to an automatic transmission, particularly to an automatic transmission with multiple gear ranges.
Automatic transmissions installed in vehicles (e.g., automobiles) generally include planetary gear sets (planetary gear mechanisms) and a plurality of friction fastening elements, and multiple gear ranges are attained by selectively fastening the friction fastening elements. Recently, multiple gear ranges are provided in transmissions to improve fuel consumption, etc. For example, WO2013/117369A1 (FIGS. 1 to 5) discloses an automatic transmission having eight forward gear ranges by including four planetary gear sets, three clutches, and two brakes.
The planetary gear sets are disposed on an input shaft. Therefore, a dimension of the automatic transmission in its axial direction (the axial length of the automatic transmission) becomes longer as the number of the planetary gear sets increase due to providing multiple gear ranges, which degrades the installability of the transmission in vehicles.
Thus, as disclosed in JP2010-048318A (FIG. 3), it is proposed to coaxially arrange a plurality of clutches in layers in a radial direction of an automatic transmission. In this manner, the axial length of the transmission becomes shorter compared to a case where all the clutches are aligned in the axial direction.
With the automatic transmission in JP2010-048318A, friction plates of two clutches are arranged in layers in the radial direction without overlapping with each other in an axial view of the automatic transmission (i.e., with a gap therebetween in the radial direction). However, although pistons for pushing the friction plates are arranged in layers in the radial direction, they are partially overlapped with each other in the axial view, in other words, they are also aligned in the axial direction. Further, operational hydraulic chambers into which hydraulic pressure for pushing the pistons is supplied, and operational hydraulic passages for supplying the hydraulic pressure into the operational hydraulic chambers are also provided in the axial direction. Therefore, there is a disadvantage that the pistons and the operational hydraulic passages inhibit shortening the axial length of the transmission, and the dimension of the automatic transmission in the axial direction cannot sufficiently be shortened.